1. Field of the Invention
The present invention relates to a water-based ink suitable for use in ink-jet recording in which an ink is ejected and flown as droplets from an ejection opening (orifice), and these ink droplets are applied to a surface of a recoding medium, thereby conducting recording, and an ink-jet recording method and instruments using such an ink, and particularly to an ink-jet ink capable of preventing occurrences of feathering and bleeding and forming images high in color strength, and an ink-jet recording method and instruments using such an ink. According to the present invention, additional mechanisms required to enhance a recording performance of the instruments are also simplified, so that increase in cost is effectively checked, and at the same time, energy saving can be achieved.
2. Related Background Art
Water-based inks have heretofore been principally used as ink-jet recording inks from the viewpoint of safety, odor and the like. There have been known inks in which one or more of various water-soluble dyes or pigments are used as coloring materials and dissolved or dispersed in water or a mixed solvent of water and a water-soluble organic solvent, and a humectant, a dye-dissolving aid, a mildewproofing agent and/or the like are added thereto as needed. For the past few years, ink-jet recording using such inks has conspicuously spread because it has such many advantages as the inks can be ejected in a proportion of several thousand droplets per second to conduct high-speed recording with ease, noise is scarcely produced, multi-color recording can be performed with ease, high-resolution recording can be effected, and recording can be conducted on plain paper.
With the development of low-cost and high-performance personal computers and the standardization of GUI environment in recent years, even image recording by printers or the like has been required to achieve high coloring, high quality, high fastness properties, high-resolution recording, high-speed recording and the like. In response to this requirement, various technical ideas that a coloring material component is left on the surface of paper as much as possible to increase the optical density of images to be formed, edges of printed dots are made sharp, and the occurrence of feathering, bleeding and the like is lessened are also being proposed in ink-jet recording.
As a first example thereof, Japanese Patent Application Laid-Open No. 58-13675 discloses a method of controlling an absorption and spreading of recording dots in and on paper by adding polyvinyl pyrrolidone to an ink. As a second example thereof, Japanese Patent Application Laid-Open No. 3-172362 discloses a method of controlling the absorption of an ink and the spreading of dots by adding a specific microemulsion to the ink.
As a third example in which a sol-gel transition phenomenon is applied to an ink, Japanese Patent Application Laid-Open Nos. 62-181372 and 1-272623, etc. each describe an ink which is in a gel state at room temperature, but turns to a sol state by heating. It is said that according to such an ink, a penetration of the ink into paper can be controlled because recording on a recording medium is conducted in a sol state, and the ink turns to a gel state by its cooling.
As a fourth example, Japanese Patent Application Laid-Open No. 6-49399 has recently disclosed an ink, to which a compound having reversibly and thermally gelling property is added, and which has a good coloring ability and a fixing ability, causes little feathering, provides prints excellent in shelf stability, and is also excellent in reliability, and an ink-jet recording method and instruments using such an ink. The technical background thereof is based on a phenomenon that when an aqueous solution of a specific water-soluble polymer is gradually heated, its water-solubility is lowered, and so the solution becomes cloudy (a temperature at which such a phenomenon occurs is called xe2x80x9cclouding pointxe2x80x9d).
Typical examples of the water-soluble polymer include poly(N-isopropylacrylamide), poly(vinyl methyl ether), polyethylene oxide and hydroxypropylcellulose. Since these polymers have a negative temperature coefficient as to solubility, they are in a state separated and deposited from a solution at a temperature not lower than the clouding point. In such a deposited state, hydrophobic microgel is formed, and a viscosity of the solution decreases. When recording is conducted on a recording medium in a deposited state, the viscosity of the solution returns to the initial viscosity, i.e., increases, owing to the temperature drop on the recording medium, and so the penetration of the ink can be prevented.
As a fifth example, M. Croucher et al. have indicated problems involved in the conventional homogeneous inks and moreover proposed, as a future ink for ink-jet, a heterogeneous ink making good use of a latex [M. D. Croucher and M. L. Hair; xe2x80x9cDesign Criteria and Future Directions in Inkjet Ink Technologyxe2x80x9d, Ind. Eng. Chem. Res., 28, 1712-1718 (1989)].
U.S. Pat. No. 4,246,154 discloses an ink in which fine particles of a vinyl polymer are colored with a dye and anionically stabilized. U.S. Pat. No. 4,680,332 also discloses a heterogeneous ink in which a water-insoluble polymer containing an oil-soluble dye and combined with a nonionic stabilizer is dispersed in a liquid medium. Further, in U.S. Pat. No. 5,100,471, there has been proposed a water-based ink comprising a solvent and colored particles composed of a polymer core and a silica shell to which a dye has been bonded by covalent bonding. The patent describes this ink as having such features as it produces a brighter color on paper, is stable to temperature change, and provides images high in water fastness.
On the other hand, as a sixth example, it has been proposed in Japanese Patent Application Laid-Open No. 3-240586 to use, as a nonaqueous ink, an ink in which colored particles coated with a resin which swells with a dispersion medium are dispersed in kerosene or the like. In this proposal, it is said that the ink is effective in, particularly, prevention of image feathering and of clogging at an orifice for ejecting ink droplets.
As a seventh example, further, Japanese Patent Application Laid-Open No. 63-87279 discloses, as a recording means for permitting a provision of high-quality images free of any feathering irrespective of paper, an ink-jet recording method in which an ink applied to a recording medium is heated at a temperature (80 to 140xc2x0 C.) higher than room temperature in such a manner that physical property values of the ink become the desired values depending upon the combination of the recording paper and the ink.
However, the above-described first and second examples of the prior art involve a problem of the fixing ability of the ink in that since the penetration into paper is prevented, the ink does not penetrate into the paper, but remains on the paper for a long time. A problem that color mixing (bleeding) between inks of different colors also arises.
The ink based on the sol-gel transition of the third example involves a problem that running behavior may arise due to changes in storage temperature of the resulting prints, and so staining by color mixing and transfer due to running of images occurs.
The ink containing the reversibly and thermally gelling compound of the fourth example is unfit for a recording method in which one pixel is recorded at a high speed in several tens of milliseconds or shorter like ink-jet recording because it uses a water-soluble cellulose ether, and so its viscosity increase is slow. If the ink is used in ink-jet recording, such a compound must be used in a low concentration because the upper limit of viscosity upon ejection of the ink is as low as at most 20 mpaxc2x7s. It is hence difficult to sufficiently achieve a thickening effect.
On the other hand, among the fifth example group, the ink in which a coloring material has been anionically stabilized involves a problem that a pH region in which the coloring material is stably dispersed is narrow, and a selection range of dyes is hence limited. In addition, it has a disadvantage that printed dots thereof are small in spreading on paper, and an optical density (OD) required is hence hard to achieve. With respect to a reduction of fixing time, which is required for high-speed recording, the ink has little effect because fixing depends on only evaporation and penetration like the conventional image-forming means.
According to another disperse ink comprising a polymer containing an oil-soluble dye and combined with a nonionic stabilizer, the range of selection of dyes can be widened. As with the above ink, however, this ink has little effect on the shortening of fixing time because fixing is based on a mechanism depending on evaporation and penetration. In addition, the ink is disadvantageous even from the viewpoint of color mixing (bleeding) between inks of different colors because it takes a lot of time for fixing between adjacent dots.
A disperse ink of the polymer core/silica shell structure is excellent in dispersion stability of a pigment, but does not provide a sufficient OD because the ink has no particular means for an aggregation of a coloring material on a surface of paper. In addition, the ink has little effect on a reduction of fixing time because fixing depends on evaporation and penetration. Therefore, the ink involves a problem that bleeding occurs.
As a problem common to the above three examples, may be mentioned a problem that a rub-off resistance of recorded images is poor because an adhesion of a coloring material to the surface of paper is not taken into consideration in such inks.
The sixth example in the prior art involves problems of odor, safety and the like because it uses kerosene as a dispersion medium.
In the seventh example, recording is effected by heating a surface of paper to at least 80xc2x0 C. and making good use of changes of physical properties of the ink applied to paper by heat. Since the temperature of the ink is raised by secondary heating through paper, the method involves a problem that the quantity of heat required for a temperature rise is as great as 1 cal/g.xc2x0C. or higher, and so the method is not efficient. In addition, the method also involves a disadvantage that when the temperature must be raised to 100xc2x0 C. or higher, a passageway for discharging steam generated must be provided, and so the apparatus used must be of a large size. Further, the method is accompanied by the problem that the ejecting ability of a recording head is deteriorated due to evaporation of water in the ink within an nozzle of the recording head caused by radiant heat from the paper heated, and viscosity increase consequent thereon.
To eject ink droplets, water-based inks, particularly ink-jet inks, are required to have such physical properties as a surface tension not greater than 20 dyn/cm (related to refilling speed), a viscosity within a range of from 1 to 20 mpaxc2x7s, a pH value of about 3 to 10, and a fixing time shorter than 20 seconds (it is better to be the shortest possible time).
Here, a transfer of ink to paper is considered. With respect to the transfer phenomenon of liquid to paper, the Lucas-Washburn equation has been generally known. Supposing an amount of the liquid transferred, a roughness index of the paper, an absorption coefficient, transferring time and wetting-starting time are V, Vr, Ka, T and Tw, respectively, in case the liquid is water, the amount V of the transferred liquid is represented by the equation
V=Vr+Ka{square root over (Txe2x88x92Tw)}xe2x80x83xe2x80x83(1).
In the equation (1), Ka is related to physical properties of both paper and ink and is represented by the equation
Ka={square root over (r+L xc2x7xcex3 cos xcex8/2xcex7+L )}xe2x80x83xe2x80x83(2)
wherein r is a capillary radius of the paper fiber, xcex3 is a surface tension of the liquid, xcex8 is a contact angle, and xcex7 is a viscosity of the liquid.
It is understood from the equation (1) that in order to leave a coloring material on a surface of the paper, it is necessary to delay a penetration of the liquid, namely, make the Ka value small, as far as possible (the time of evaporation can be gained by making Ka small). It is understood from the equation (2) that it is only necessary that the surface tension xcex3, the viscosity xcex7 and the contact angle xcex8 be made low, high and great, respectively, as physical properties of an ink required to do so. However, the physical properties of the ink-jet ink are variously limited as described previously. It is thus difficult to control Ka.
On the other hand, when a nonaqueous solvent, for example, ethanol, is used as the liquid, the wetting-starting time Tw in the equation (1) may be ignored. Therefore, fixing can be accelerated. However, the Ka value also becomes greater, and so a penetrating effect is increased, resulting in an image on which xe2x80x9cfeatheringxe2x80x9d occurs to a great extent. In addition, the term of cos xcex8 in the equation (2) is determined by the combination of ink and paper. Therefore, whether the quality of images is good or poor depends on the kind of paper used. Namely, such an ink cannot satisfy paper nondependence.
It is considered that the above-mentioned problems may arise even on the conventional coloring material-dispersed inks so far as their image formation depends on penetration and evaporation.
It is accordingly an object of the present invention to provide an ink-jet ink which can solve the above problems involved in the prior art, in particular, can prevent occurrences of feathering and bleeding and form images high in color strength, and an ink-jet recording method and instruments using such an ink.
The above object can be achieved by the present invention described below.
According to the present invention, there is thus provided a water-based ink for ink-jet, comprising a liquid composition containing a coloring material and a liquid medium, wherein the ink comprises a polymer exhibiting a heat-reversible type thickening property and a compound having a light-heat converting function.
According to the present invention, there is also provided a water-based ink for ink-jet, comprising a liquid composition containing a coloring material and a liquid medium, wherein the ink comprises a polymer which exhibits heat-reversible type thickening property and has a moiety capable of undergoing light-heat conversion.
According to the present invention, there is further provided an ink-jet recording method comprising ejecting droplets of an ink, which comprises a coloring material and a liquid medium, out of an orifice of a recording head in accordance with recording signals to apply the droplets to a recording medium, thereby conducting recording, wherein said ink is any one of the inks described above.
According to the present invention, there is still further provided an ink-jet recording method comprising ejecting droplets of an ink, which comprises a coloring material and a liquid medium, out of an orifice of a recording head in accordance with recording signals to conduct recording on a recording medium, wherein said ink is any one of the inks described above, and the recording head is kept at a temperature not higher than a transition temperature of the ink during the recording.
According to the present invention, there is yet still further provided a recording unit comprising an ink container portion containing an ink therein and a recording head from which the ink is ejected in the form of ink droplets, wherein said ink is any one of the inks described above.
According to the present invention, there is yet still further provided an ink-jet recording apparatus comprising a recording unit equipped with an ink container portion containing an ink therein and a recording head from which the ink is ejected in the form of ink droplets, wherein said recording unit is the recording unit described above.